The United Nations human rights chief Zeid Ra'ad al-Hussein called on China Tuesday to immediately and unconditionally release detained lawyers and activists.

Since last July, China has detained about 250 "human rights lawyers, legal assistants and activists," according to a statement from Zeid's office. Many of those detained have since been released but the U.N. official says the arrests represent a "worrying pattern" in China.

The United Nations and China have been in constant contact regarding the issue, according to Rupert Colville, a spokesman for Zaid's office. Colville said China's response letter accused those arrested of criminal activities and did not "address the core issues."

Zeid expressed concern about the trend of governments cracking down on dissenters.

"Lawyers should never have to suffer prosecution or any other kind of sanctions or intimidation for discharging their professional duties," Zeid said.

He also expressed concern about the January disappearance of five Hong Kong employees of a publishing house that had released works critical of the Chinese government and the arrest of a Swedish civil society worker offering legal assistance in China.